Sludge discharged from a water treatment plant or a sewage treatment plant contains 90% or more water. Thus, as is conventionally done, the sludge is concentrated in a gravity settling tank and thereafter mechanically dehydrated to obtain thickened sludge having a water content of 80% or lower. This thickened sludge is incinerated and disposed of at landfill.
In the conventional sludge concentration apparatus using a gravity settling tank, a long retention time is required and the water content fluctuates significantly according to the sludge property. It is difficult to stably perform mechanical dehydration. For this reason, a filtration concentration apparatus has been developed as an alternative to the gravity settling tank. Referring to FIG. 10, such a conventional filtration concentration apparatus has a sludge tank 1 having a tapered bottom part, a pipe, a valve 8 and a pump 2 for supplying raw sludge to the sludge tank 1, a water level gauge 18 for detecting the level of the sludge to determine the sludge level (so that a predetermined level can be achieved by activating the pump 2 when supplying the raw sludge to the sludge tank 1), a pipe and a valve 9 for discharging the thickened sludge at the lower portion of the sludge tank 1, a plurality of sludge plates 3 disposed within the sludge tank 1, a filtrate discharge pipe 4 for discharging the filtrate obtained after filtering out the sludge using the filter plates 3 (only one illustrated), a feed tank 5 connected to the middle of the filtrate discharge pipe 4, a tap water supply pipe 6 and a valve 12 for supplying water to the feed tank 5, a pipe and a valve 14 for discharging the air accumulated within the feed tank 5, and a compressor 7 and a valve 13 connected to the feed tank 5 to supply pressurized air. The filtrate discharge pipe 4 is installed such that the outlet thereof is positioned below the bottoms of the filter plates 3. Also, the filter plates 3 each are constituted by a filtration film or membrane 3a that isolates the filtrate from the sludge, and a filter frame 3b that supports the filtration membrane 3a. 
FIG. 11 illustrates the conventional configuration of the filter plate 3. An air distribution pipe 15 for drawing the filtrate or separating the thickened sludge is provided in an upper portion of the inside of the filter plate 3. The air distribution pipe 15, which is obtained by installing a round- or angular-shaped pipe made of metal or polyvinyl chloride in a horizontal direction, has a plurality of holes 16 directed in a vertically downward direction at a bottom portion thereof. Each hole 16 is provided to draw the filtrate within the filter plate 3 or to supply pressurized air that separates thickened sludge from the filtration membrane 3a. The end on the side opposite to a pressurized air blowing port of the air distribution pipe 15 is sealed.
Next, the method of operating the conventional filtration concentration apparatus is described. The valves 9 through 14 are kept closed while the valve 8 is left opened. Then, the pump 2 is activated to supply raw sludge up to a predetermined level at which the filter plate 3 installed in the sludge tank 1 is submerged.
Next, the valves 12 and 14 are opened to supply water up to a predetermined water level in the feed tank 5, and thereafter the valves 12 and 14 are closed. Then, the valves 10 and 11 are opened to begin filtration. The pump 2 is activated such that the level of the raw sludge is kept at this predetermined level during the filtration process. Supply and filtration of the sludge are continued until the thickness of the sludge deposited onto the filtration membrane 3a becomes, preferably, 10 through 13 mm. Regarding the time for continuing the filtration, the relationship between the thickness of the deposited sludge and a filtration time period can be studied beforehand, and then the filtration can be conducted with reference to the filtration time period during which the thickness of the deposited sludge becomes 10 through 13 mm. For example, in the case of this siphon filtration, filtration is performed for 90 minutes.
Next, the valve 10 is closed, while valve 8 is opened and the pump 2 is activated. The sludge that is not adhered to surfaces of the filtration membrane 3a is discharged from the sludge tank 1. After the sludge is discharged from the sludge tank 1, the valve 8 is closed. A primary side of the filtration member 3a is the side where the sludge to be treated or filtered contacts therewith, and a second side of the filtration member 3a is the side where the treated or filtered sludge contacts therewith. Next, the valves 10 and 13 are opened, and air is introduced to the secondary side of the filtration membrane 3a by means of a compressor 7 and pressurized. Accordingly, in a state in which the water within the feed tank 5 and the filtrate discharge pipe is mixed with the air from the compressor 7, the thickened sludge that is adhered to the surface of the filtration membrane 3a on the primary side is dropped from the filtration membrane 3a and is deposited onto the bottom of the sludge tank 1. Then, the valve 9 is opened to discharge the thickened sludge from the sludge tank 1.
The principle of removing the thickened sludge using the filter plate is summarized as follows.                The pressurized air supplied to the air distribution pipe 15 is emitted from the holes 16 provided on the distribution pipe toward the secondary side of the filtration membrane 3a on which the thickened sludge is adhered (some of the pressurized air being supplied to filtration membrane paths). In this manner, the thickened sludge that is adhered to the surfaces of the filtration membrane 3a around the air distribution pipe is dislodged.        The removed thickened sludge is dropped along with the rest of the thickened sludge toward the lower portion of the sludge tank 1.        
The conventional filtration concentration apparatus having an air distribution pipe, however, has a problem in that the thickened sludge remains in the upper portion of the filter plate. Consequently, the amount of sludge to be processed per hour is reduced and the life of the filter plate is reduced. In other words, if the filtration cycle is repeated, further filtration and suction are performed at the section where the sludge remains, where the adhesion force between the filtration membrane and the thickened sludge increases, causing a clogging problem.
Therefore, there remains a need for a sludge filtration apparatus capable of preventing the thickened sludge from remaining in the upper portion of the filter assembly. The present disclosure addresses this problem.